5.1.1 How fast? Flashcards
What defines the rate of reaction?
- the change in concentration of a substance in unit time
- Its usual unit is mol dm-3s-1
What do the orders of a reaction tell you? What is an individual and overall order?
- tells you how much concentration of a reactant affects the rate
- INDIVIDUAL: power to which conc is raised
- OVERALL: sum of individual orders
What are the 4 rules to remember for the rate equation?
- links rate to reactant concentrations
- only found in actual experiments
- cannot be found by looking at the stoichiometric equation
- can contain substances not in balanced eq e.g. catalysts
r = k [substance]. What do the things stand for and what are their units?
r = rate: mol dm-3s-1 USUALLY
k= rate constant - units depend on equation
[…] = concentration: mol dm-3
How do you calculate the units for k?
- rearrange to make k the subject
- substitute units you have into expression
- cancel common units and write what you have leftover
1st order: s-1
2nd order: mol-1dm3s-1
3rd order: mol-2dm6s-1
How do you use initial rates data to calculate k?
- Use changes in initial rates and compare to form the rate equation
- Substitute values in to calculate k
What are the methods of continuous monitoring?
- gass collection or mass loss
- OR colour change using colorimeter where the amount of light absorbed is measured
What are the graph shapes for concentration-time for 0 order and 1st order? What is the gradient representing?
- straight line with negative gradient = 0 order (grad=k)
- curved line with decreasing gradient = 1st order.
- gradients of both = the rate
What is special regarding half-life for 1st order reactions?
it is constant = exponential decay
How do you determine k for first order reactions?
- from rate -> draw tangent at a conc and calculate the gradient. rearrange for k and substitute conc values in.
- from half life -> ln2/half life equation. This is also more accurate.
What do the rate-concentration graphs look like for 0 order, 1st order and 2nd order reactions?
- 0 order: horizontal line
- 1st order: straight line with a gradient that is k
- 2nd order: a curved line -> by plotting a 2nd graph of rate against concentration squared = straight line = grad of k.
What is the initial rates method?
CLOCK REACTION
- time from the start of an experiment is measured for a visual change to occur.
- assumed the average rate over this time is the same as the initial rate.
- 1/t then gives rate
What is a common type of clock reaction?
- iodine
- appearance and time can be measured
- orange-brown when aqueous
- starch usually added so it forms an intense blue colour
How accurate is the clock reaction?
- reasonably accurate provided less than 15% of the reaction has taken place
- when there is a shorter time period over which the average rate is measured, then the rate hardly changes and therefore the tangent is closer to the actual rate line.
- when there is a longer time period, the rate changes more and isn’t as close to the rate line, so it’s less accurate.
What is a reaction mechanism? What is the rate-determining step? What rules are there to consider?
- series of steps making an overall reaction
- slowest step
- The rate equation only includes reacting species in the RDS, other species must be in the fast step
- orders in the rate equation must match the no. of species involved in the RDS
- if the RDS is the 2nd step, then ALL reactants before this are in the rate equation unless an intermediate)
